Field of the Invention
The invention generally relates to inter-radio access technology (inter-RAT) performance enhancement, and more particularly, to mobile communications devices and methods for inter-RAT performance enhancement in a wireless communications system associated with different RATs.
Description of the Related Art
In a typical mobile communications environment, a user equipment (UE) device may communicate voice and/or data signals with one or more service networks via cellular stations of the service networks. The wireless communications between the UE and the service networks may be in compliance with various radio access technologies (RATs), such as the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA 2000) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, Universal Mobile Telecommunications System (UMTS) technology and others.
Moreover, inter-radio access technology (inter-RAT) mobility involves transfer of a connection or mobility between a UE and a network from one RAT (e.g. the LTE system) to another RAT, such as the GSM/GPRS, UMTS, or CDMA system. A UE may perform the inter-RAT procedure to move to a RAT due to certain reasons such as handover, cell redirection, cell change order, or cell reselection. Conventionally, in some cases, UE may be redirected from one network supporting a first technology to a second network supporting a different technology. For example, some LTE networks may not support voice connections, or in some cases, an operator may wish to locate a voice-oriented device on a 2G or 3G network or move a voice connection to another network under their control or coordination for various reasons, such as mobility, loading, usage type, or other reasons. Alternately, the operator may wish to move a user receiving an incoming voice call to the other network. For example, the operator may wish to redirect a user from an LTE network and associated cells to another network, such as a UTRAN or GERAN network (e.g., a network that natively supports CS connections), using a procedure known as Circuit-Switched Fallback (CSFB). After that, if the UE needs to move to a previously camped RAT, the UE may need to execute a PLMN search, cell redirection or cell reselection. In the case of a PLMN search, the UE has to stop all of the on-going procedures, start a new for a PLMN, and then search for a cell of the PLMN. However, the UE may not be able to move to the camped RAT, or it may take a lot of time to move to the camped RAT.
For example, a UE in the LTE system may fall back to a legacy system, such as UMTS and GSM, due to loss of service, cell reselection, handover, CSFB or Internet Protocol Multimedia Subsystem (IMS) Single Radio Voice Call Continuity (SRVCC). Once the UE falls back to the legacy system, it is difficult to return to the LTE due to a lack of LTE cell information provided by the legacy system.